I sail a freeformula 158 with the drake 70cm fin. I too have never found it too big. In fact it is the only fin I use on the ff158. The 64 never seemed to be enough in the marginal winds and for cranking upwind.
I would recommend upgrading to the 70 - it is worth it.
I sail it with a nx4 10.4 and a retro 8.5.